Warp Knitting

It is a commonly known fact that fabrics are constructed through two major techniques- weaving and knitting apart from other minor techniques. In these processes, two distinct sets of yarns called the warp and the weft are interlaced with each other to form a fabric. 'Warp' is the set of yarns that are laid out first on a loom or frame and 'Weft' is the yarn that is woven under and over the warp yarns that are already stretched onto the loom. Thus warp is the continuous row of yarns and the wefts are the yarns that are woven in from side to side. If we go by these definitions, it is clear that textile warping is the processing of creating the base yarn that runs top to bottom on woven cloth.

Beginning of Warp Knitting 
Warp knitting is an important and an ever growing industry. When compared to weaving, this industry can be considered as newer. Nobody knows about who invented weaving and hand knitting but it is known that mechanical knitting, in form of socks producing machine, was invented by Reverend William Lee in 1589. Crane of Nottingham applied warp yarn guides to the knitting frame invented by Lee in around 1775 which initiated warp knitting. Paget, in 1861 and Willium Cotton, in 1864, made certain improvements in the looms. The compound needle was invented by Mattew Townsend in 1849, which contributed in making the textile knitting machine simpler and faster.

Warp Knitting Machines
There are two basic types of warp knitting machines. They are- Tricot knitting machine and Raschel knitting machine. Earlier, the Tricot machines were equipped with bearded needles and Raschel machines were equipped with the compound needles. However, the modern versions of both these types of machines are equipped with compound needles. The distinction between these two machines is, therefore, made by the type of sinkers in them and the roles these sinkers play in loop formation. The sinkers in a Tricot machine control the fabric throughout the knitting cycle. However, in the Raschel machines, sinkers are only used to ensure that the fabric stays down when the needles rise. The type of knitting machine influence the product construction specifications and, therefore, is an important factor in the whole process.

Yarn Preparation 
Yarn preparation in warp knitting combines methods used in weaving and knitting. In some cases, the ends of yarn can be fed directly off cones into the knitting machine but the number of cones needed restricts this working method. The large floor space required for a creel is justified only when it is technologically essential- for example, with Jacquard and curtain machines. In all the other cases, the yarn ends are fed off warp beams. Yarn preparation can be reduced to a simple winding of yarn ends on to the warp beams in a knitting machine since artificial yarn is mainly used along with moderate tensions applied to the knitting yarn. As such, smooth operation can be ensured without sizing the yarn.

The quality of warp beam is crucial for determining the quality of the knitted fabric. Variations in yarn thickness, tension, twist and other factors too might result in a defective fabric. In most of the cases, warping mistakes are not easy rather impossible to correct during the knitting process.

Methods of Warping 
The yarn manufacturers, these days, can supply prepared warps but most of the knitting firms prefer to prepare their own warping equipment and warp beams independently. Mostly, they select the standard types of yarns and warp effect yarns in the plant. There are two basic methods of warping that can be used to prepare the warps for the knitting machines- Indirect Warping and Direct Warping

Indirect Warping: The yarns from the yarn packages are wound onto an intermediate cylinder (mill) in many parallel groups with a specified density, and then they are back wound onto the warp beam.

Direct Warping: The ends of the yarn are wrapped in one operation, from the yarn packages onto the warp beam.

However, there are certain requirements that have to be kept in mind while using both the methods, information about which has been given in the table below.

Requirements for Direct and Indirect Warping
Requirement	Yarn ends density	Direct Warping
Yarn ends per section	R	R
Number of revolutions	R	R
Warp length	R	R
Number of sections	NA	R
Yarn ends per section	NA	o
R - Required; O - Optional; NA - Not Applicable

Warping Defects 
There are certain major warping defects on beam warpers that can be identified as given below.

Lapped Ends :
The broken end of yarn is not tied to the end on the warp beam and overlaps the adjoining yarn. The beam is not properly braked and the signal hook fails to operate.

Bulges : Yarn ends are drawn from the middle and the broken end is not correctly pieced up to the adjoining yarn.

Broken ends on the beam : It occurs due to reasons mentioned in the above point. A group of ends is broken and tied as a bunch or worked-in with overlapping.

Yarn cut at the butts of the warp beam/ slackness of extreme yarns : It occurs when the reed is improperly set with respect to the warp beam flanges or there is a deformation of the warp beam flange.

Excessive or insufficient number of yarn ends : The number of yarn ends of the beam becomes excessive or insufficient due to the incorrect number of bobbins in warping.

Conical winding on the beam : 
It occurs due to incorrect load applied by the pressure roller. 

Slacks & irregular yarn tension :
It happens due to any on of these reasons- improper threading of the yarn into the tension devices, ejection of yarn from under the disc of the yarn tensioning device, or yarn tension devices of poor quality.

Frequent yarn breakages at the beam edges : 
It results due to burrs and nicks on the surface of the warp beam flanges.

Improper length of warping :
It is due to malfunction of the counter, and the brakes of the measuring device & warp beams. 

Coarse Knots : 
It is due to manual tying-up.

Loose yarn winding :
It happens when the pressure roller is lightly pressed against the warp roller.

Fluff, oily ends and yarn of different density : 
It is due to the careless work of the operater, creeler and oiler.

Bulgy winding on the warp beam :
It is due to Irregular laying of yarn ends in the reed, missing a dent and placing two ends in the adjoining yarn.